Some, if not most, health care providers in the United States will agree to bill the insurance company if patients are willing to sign an agreement that they will be responsible for the amount that the insurance company doesn't pay. The insurance company pays out of network providers according to "reasonable and customary" charges, which may be less than the provider's usual fee. The provider may also have a separate contract with the insurer to accept what amounts to a discounted rate or capitation to the provider's standard charges. It generally costs the patient less to use an in-network provider.
Bärnighausen, Till; Sauerborn, Rainer (May 2002). "One hundred and eighteen years of the German health insurance system: are there any lessons for middle- and low income countries?" (PDF). Social Science & Medicine. 54 (10): 1559–87. doi:10.1016/S0277-9536(01)00137-X. PMID 12061488. Retrieved 10 March 2013. As Germany has the world's oldest SHI [social health insurance] system, it naturally lends itself to historical analyses.
Here's what might trigger a special enrollment period: divorce, marriage, birth or adoption of a child, death of a spouse or partner that leaves you without health insurance, your spouse or partner who has you covered loses his/her job and health insurance, you lose your job and with it your health insurance, your hours are cut making you ineligible for your employer's health insurance plan, or you are in an HMO and move outside its coverage area.